FIG. 2 is a cross-sectional view showing the structure of a prior art semiconductor laser producing visible light, i.e., light visible to the naked eye. The laser includes an n-type gallium arsenide (GaAs) substrate 1 on which numerous layers are successively disposed. An n-type aluminum gallium indium phosphide (AlGaInP) cladding layer 2 is disposed on the substrate 1, an undoped indium gallium phosphide (InGaP) active layer 4 is disposed on the cladding layer 2, a p-type AlGaInP light guide layer 5 is disposed on the active layer 4, and a p-type InGaP etch stopping layer 6 is disposed on the light guide layer 5. Those layers 2, 4, 5, and 6 are all successively grown on the substrate 1 by conventional techniques, for example, by metal organic chemical vapor deposition (MOCVD). A current blocking layer 12 and mesa structure 13 are disposed on the etch stopping layer 6 to concentrate the current flow in the central portion of the active layer 4 and to form a loss guide structure for transverse oscillation mode stabilization.
To form the mesa structure and current blocking layer, initially a p-type AlGaInP cladding layer 7, a p-type InGaP transition layer 8, and a p-type GaAs contacting layer 9a are successively grown on the etch stopping layer 6. Thereafter, an etching mask is formed on part of the contacting layer 9a and the mesa 13 is formed by etching the unmasked portions of layers 7, 8, and 9a. An etchant that etches AlGaInP far more rapidly than InGaP is employed. Thus, when the etch stopping layer 6 is exposed, the rate of etching declines rapidly and damage to the underlying light guide layer 5 that would occur if the etch stopping layer 6 were absent is avoided.
The current blocking layer 12 of n-type GaAs is grown on the etch stopping layer 6 at both sides of the mesa 13. By using MOCVD or another selective growth process with the etching mask still in place on the remaining contacting layer 9a, the current blocking layer 12 does not grow on top of the mesa 13. Finally, a second p-type contacting layer 9b is grown on the top of the mesa 13 and on the current blocking layer 12 after the etching mask is removed. Electrodes 10 and 11 are formed on the substrate 1 and the second contacting layer 9b, respectively, to complete the semiconductor laser. Either before or after the electrodes are formed, the structure is cleaved to form the opposed facets of the laser.
The prior art laser structure is described above without specification of the relative concentrations of the various elements in the ternary and quaternary alloys. The undoped active layer 4 in a semiconductor laser producing visible light typically is usually approximately Ga.sub.0.5 In.sub.0.5 P. The light guide layer 5 and the cladding layers 2 and 7 are usually approximately Al.sub.0.28 Ga.sub.0.23 In.sub.0.49 P. Likewise, in the structure of FIG. 2, the transition layer 8 is intended to have an energy band gap intermediate those of the layers 7 and 9a to reduce the voltage drop that occurs when the GaAs contacting layer directly contacts the AlGaInP cladding layer. In this laser structure, transverse mode oscillation is stabilized by a loss guide structure. Light produced in the active layer 4 that reaches the current blocking layer 12 is absorbed because of the smaller energy band gap of GaAs. Within the mesa 13, the light is not absorbed because of the larger energy band gap of the AlGaInP cladding layer 7. The loss guide structure concentrates light at the mesa 13, stabilizing the oscillation mode of the laser.
The conductivity types of the layers of the laser of FIG. 2 are determined during growth by including appropriate dopant impurities in the growing layers. For example, the dopant impurity that typically provides p-type conductivity in the light guide layer 5 and the cladding layer 7 is zinc (Zn). The dopant impurity used to produce n-type conductivity in the cladding layer 2 is typically selenium (Se) or silicon (Si). When Zn is employed as a p-type dopant impurity in AlGaInP, e.g., in the light guide layer 5 and the cladding layer 7, it has been observed to have a relatively low degree of electrical activity. In other words, a relatively small proportion, for example, only forty percent, of the incorporated Zn atoms are ionized and act as acceptors. The remainder of the Zn does not affect the electrical properties of the laser. To compensate for that low degree of ionization, a relatively large amount of Zn is incorporated into the growing layers. The diffusion coefficient of Zn in AlGaInP is larger than that in GaInP. As a result of a relatively high concentration of Zn and the relative diffusion coefficients of Zn in the cladding and active layers, unusual dopant impurity concentrations can occur in the laser structure. Those dopant impurity concentration abnormalities are accentuated by a known interaction between Zn and Se.
The unusual dopant impurity concentrations that can occur in the laser structure of FIG. 2 are illustrated in FIG. 3. There, the relative concentrations of Zn and Se in the cladding layer 2, the active layer 4, and the light guide layer 5 are plotted as a function of position. The expected concentration of Zn in the layer 4, absent the high concentration of Zn in the light guide layer 5 and the different diffusion constants of the layers 2 and 7, is illustrated by a broken line. Because of the difference in diffusion coefficients of Zn in the active layer 4 and the light guide layer 5, an abnormally large concentration of Zn can occur near the interface of the active layer 4 and the light guide layer 5, as shown in FIG. 3. The increased Zn concentration and the elevated temperatures employed in growing the various layers of the laser structure cause the Zn to diffuse into the active layer 4 that is desirably neither n-type nor p-type. The abnormally high Zn concentration at the interface effectively provides a high concentration diffusion source, accelerating the intrusion of Zn into the active layer 4. The Zn diffusion can occur during growth of the layers of the semiconductor laser, during other high temperature process steps in the fabrication of the laser, or during operation at elevated temperatures. When the concentration of Zn in the active layer 4 increases, undesired charge carrier recombination occurs in that layer, reducing the light output of the semiconductor laser. To compensate for the reduced light output, the current flowing through the laser may be increased, increasing the operating temperature of the laser, accelerating further Zn diffusion and premature failure, i.e., shortened lifetime, of the laser.